offset_converter: minor refactoring
offset_converter: minor refactoring

DRY: extract method for `joint_angle` calculation.

mapnik/mapnik,lightmare/mapnik,naturalatlas/mapnik,tomhughes/mapnik,naturalatlas/mapnik,mapnik/mapnik,lightmare/mapnik,mapnik/mapnik,tomhughes/mapnik,lightmare/mapnik,naturalatlas/mapnik,naturalatlas/mapnik,lightmare/mapnik,tomhughes/mapnik,mapnik/mapnik,tomhughes/mapnik